System and method for focal length stabilization using active temperature control

ABSTRACT

An optical metrological system having a heat-generating light source coaxially mounted near a heat-sensitive lens. The system uses a temperature sensor to monitor the lens temperature and a heating element to heat the lens such that the lens operating temperature is greater than a maximum operating temperature of the light source in order to stabilize the focal length of the lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/185,632, filed Aug. 4, 2008, now U.S. Pat. No. 7,602,563. which is acontinuation of No. 11/633,303 filed Dec. 4, 2006, now U.S. Pat. No.7,408,728

BACKGROUND AND SUMMARY

This invention is related to metrological methods and systems, and moreparticularly, to methods and systems for focal length stabilization ofmetrological systems using active temperature control.

Automated metrology systems are used for the optical inspection of anobject. Such inspections are performed in order to obtain precisedimensional and other measurements of the object. The object is placedon a stage (with precision movements for X-Y measurements) and the imageof the object undergoes computerized image analysis. The Z-axis is alsomeasurable using the auto-focus routine of the software resulting inheight measurements of the object. A precise three-dimensionalreproduction of the object is obtained using the measurements. Theoptical assemblies used in such systems are composed of a main imagingpath, a calibration system and a surface illumination system forillumination of the object to be inspected.

The main imaging system of an optical system used for metrology, asdescribed, for example, in U.S. Pat. No. 6,292,306, is comprised of afront lens, and either a fixed or zoom system behind, with a camera inthe focal plane of the system. The system is designed to have collimatedspace between the front lens and the zoom or fixed lens portion of theimaging system. Z-axis measurements are taken with auto-focus, acomputerized image analysis, to find the best focus of the system.Because of the collimated space behind the front lens, the front focusof the system is ultimately the front focus of the front lens. Anyenvironmental changes to the front lens that may cause the physicalfront focus to change will add error to the Z-axis measurement of theobject.

The calibration system, as described, for example, in U.S. Pat. No.5,389,774, allows a user to perform calibration and return to apreviously saved magnification. This is done by saving a reticle imageat a selected magnification, calling up that image when thatmagnification is desired again, and waiting for the zoom lens to adjustuntil the present reticle image matches the saved reticle image.

The surface illumination system uses a variety of techniques toilluminate the object to be measured in order to enhance the precisionof the measurements. One technique uses an LED ring surface illuminator,as described, for example, in U.S. Pat. No. 5,690,417, that allows forcontours, ledges, edges, and other generalized surface height variationsto be imaged. In such a system, the light source may surround the frontlens of the main imaging system, creating a large amount of heatadjacent to the lens. When heated, the properties of the glass change,thus causing a change in front focal length directly affecting the frontworking distance/front focus (Z-axis measurement/height) of the system.The problem this creates is inaccurate Z-axis position measurements ofthe object to be measured. The Z-axis position measurement of the objectwill change once the light source is turned on, and will continue tochange over time, as the light source heats up, until the light sourcereaches its maximum operating temperature. Any time the light is turnedoff and the temperature of the optics is allowed to change, the Z-axisposition measurement of the object will also change. The fluctuation oflens focal length with temperature also results in repeatability errorsas confirmed by repeated measurements of the same object over a periodof time. Typically, Z-axis position measurements may fluctuate by 10-20microns due to a temperature change, depending on the optical system andthe amount of heat generated by the light source. An advantage ofembodiments is the reduction of Z-axis position measurement variationsin an object measured by a metrology system by reducing the temperaturefluctuation in a heat-sensitive lens.

Embodiments stabilize the lens temperature of an optical system having aheat-sensitive lens in proximity to a heat generating device such as alight source. Embodiments also insulate the lens from environmentaltemperature variations that can affect Z-axis position measurements.Embodiments can be used in any application in which a stable lenstemperature can optimize system performance. Embodiments provide anadvantage in that they enable accurate Z-axis position measurements ofobjects and ensure that such measurements do not vary over time.Maintaining the lens at a constant temperature can also protect the lensfrom damage such as de-cementing which is caused by sudden and frequentlens temperature variations.

An optical system with focal length stabilization in accordance withembodiments includes a housing supporting a heat-sensitive lens withinthe housing, a light source secured to the housing, a heating elementconnected to the housing to heat the lens, at least one temperaturesensor connected to the housing, and a controller in electricalcommunication with the at least one temperature sensor and the heatingelement such that the controller monitors a temperature of the lens andadjusts a current in the heating element to maintain the temperature ofthe lens within a pre-selected range.

A method for stabilizing focal length in a heat-sensitive lens supportedin a housing having a light source secured thereto in accordance withembodiments includes the steps of monitoring a lens temperature using atleast one temperature sensor connected to the housing, and maintainingthe lens temperature within a pre-selected range by controlling aheating element attached to the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of an optical system having a lightsource coaxially mounted on one end of its objective lens system andusing a temperature sensor and heating element for active focal lengthstabilization in accordance with embodiments.

FIG. 2 is a bottom view of an optical system having a light sourcecoaxially mounted on one end of its objective lens system in accordancewith embodiments.

FIG. 3 is a schematic block diagram of a temperature control system forstabilizing focal length in an objective lens system in accordance withembodiments.

FIG. 4 is a side elevation view of an optical system having a lightsource coaxially mounted on one end of its objective lens system andusing multiple temperature sensors, a heating element, and an indicatorfor active focal length stabilization in accordance with embodiments.

FIG. 5 is a schematic flow diagram of a method for stabilizing focallength in a heat-sensitive lens in accordance with embodiments.

FIG. 6 is a bottom view of an optical system having a light sourcecoaxially mounted on one end of its objective lens system in accordancewith another embodiment of the invention.

FIG. 7 is a schematic block diagram of a temperature control system forstabilizing focal length in an objective lens system in accordance withanother embodiment of the invention.

FIG. 8 is a schematic block diagram of a temperature control system forstabilizing focal length in an objective lens system in accordance withanother embodiment of the invention.

DESCRIPTION

A system 10 and method for stabilizing focal lengths in a heat sensitivelens in accordance with embodiments is illustrated in FIGS. 1-5. Thesystem 10 includes a cylindrical lens housing 12 containing aconventional objective lens system 30, a lamp supporting housing 14, atemperature sensor 16, a heating element 18, a controller 20, and alight source 22, although the system 10 can comprise other numbers andtypes of components in other configurations. The method describesstrategically heating the housing and maintaining the temperature of aheat-sensitive lens within a pre-selected range in order to stabilizethe focal length of the lens. Embodiments provide a number of advantagesincluding the reduction of inaccurate measurements and repeatabilityerrors caused by heat-induced variations in lens focal length.

Referring to FIGS. 1 and 2, the system 10 includes a cylindrical lenshousing 12 containing a conventional objective lens system 30, adisc-shaped lamp supporting housing 14, a temperature sensor 16, aheating element 18, and a controller 20 that adjusts current to theheating element to increase and decrease temperature measured at thetemperature sensor. In the exemplary embodiment shown in FIGS. 1 and 2,an annular, generally disc-shaped lamp supporting housing 14 is securedto and surrounds the lower end of the lens housing 12. A light source 22is mounted in the lamp supporting housing 14. Preferably, the lightsource comprises a plurality of lamps L, although any suitable type oflight source can be used. The lamps L are secured or mounted at theirinner ends in the lamp supporting housing 14, and the lamps L project attheir outer, light emitting ends downwardly from the housing 14 in thedirection of an object 24, which object 24 rests on a work table 26. Asshown more clearly in FIG. 2, in embodiments the lamps L are mounted inthe lamp supporting housing 14 in five circular arrays or rings disposedcoaxially of the axial centerline of the housings 12 and 14. The lamps Lare located proximate to and typically surround the lens 30 and cancreate large amounts of heat in the area around the lens 30. The lens30, which is corrected for color aberrations, is sensitive to heat. Whenheated, the focal length of the lens 30 changes with temperature,directly affecting the front working distance and front focus of thesystem, which can result in distortion of the perceived Z-axis distance.

Referring to FIG. 3, a temperature control system preferably includesthe controller 20, heating element 18, and temperature sensor 16. Thecontroller 20 includes a memory 80, a processor 82, an input/output unit84, and an indicator 86, which are connected together by a bus 88 orother link, although other suitable types and numbers of components inother configurations and other types of processing systems can be usedfor the controller. In alternative embodiments, all of the componentsare placed on a single microchip or semiconductor device. The memory 80can store instructions and data for performing one or more aspects ofembodiments, including the methods described with references to FIGS.1-5, although some or all of these instructions and data can be storedelsewhere. A variety of different types of memory storage techniques,such as a random access memory (RAM), a read only memory (ROM), flashROM, EEPROM, and the like, and even hard disk drives, can be used by thememory 80 to store the instructions and data.

Referring to FIGS. 1, 2 and 3, the heating element 18 comprises heattape, preferably of the OMEGA® KAPTON type of insulated flexible heaters(catalog no. khlv 0502/5-p), although other suitable types of heatingelements can be used. The heating element 18 is preferably wrappedaround the circumference of the cylindrical lens housing 12 as closelyas possible to the lens 30, although other locations and techniques forattaching the heating element 18 can be used. The temperature sensor 16comprises a thermocouple, although other types of temperature sensorscan be used, and is preferably mounted on the cylindrical lens housing12 as closely as possible to the lens 30, although other locations forattaching the temperature sensor 16 can be used. The temperature sensor16 is preferably attached with adhesive, though other attachmenttechniques can be used.

Referring again to FIG. 3, the controller 20 in embodiments isoperatively connected to the heating element 18 and temperature sensor16 by wire, although other techniques for connecting the devices may beused, such as wireless communications techniques. The temperature sensor16 preferably transmits a temperature measured proximately to lens 30 orsignal representative thereof to the controller 20 which, in turn,increases or decreases current to the heating element 18 as required tomaintain the temperature of the lens 30 within a target temperaturerange in accordance with methods disclosed herein.

Referring to FIG. 4, other embodiments for stabilizing focal lengths ina heat sensitive lens will now be described. The system 50 ofembodiments includes a cylindrical lens housing 52 containing aconventional objective lens system 54, a disc-shaped lamp supportinghousing 56, a light source 58 in the lamp supporting housing 56,temperature sensors 60, 62, and 64, a heating element 66, an indicator86, and a controller 70. In such embodiments, the controller 70preferably calculates a weighted average temperature of the lens system54 based on inputs from temperature sensors 60, 62, and 64, althoughother calculating methods can be used. The controller 70 of embodimentsthen adjusts current to the heating element 66 as described herein untilthe temperature measured at the lens system 54 by temperature sensors60, 62, and 64 falls within the target temperature range. The entiretarget temperature range is preferably greater than the maximumoperating temperature of the light source 58 to minimize temperaturefluctuations, and hence minimize focal length-drift in the lens system54. The indicator 86 is preferably illuminated when the temperaturemeasured at the lens system 54 is within the target temperature range inorder to advise a user that the focal length of the lens system 54 hasachieved the desired stability.

Referring to FIGS. 1, 3, and 5, these figures illustrate an example of amethod for stabilizing focal length in a heat-sensitive lens by heatingthe lens to a temperature that is greater than the maximum operatingtemperature of a light source surrounding the lens in accordance withembodiments. The method preferably comprises monitoring the temperatureof the lens 30 using the temperature sensor 16 and maintaining thetemperature of the lens 30 within a pre-selected range that is greaterthan the maximum operating temperature of a light source 22 surroundingthe lens by controlling a heating element 18 attached to a cylindricalhousing 12 surrounding the lens 30. The pre-selected range inembodiments comprises a minimum and maximum temperature that can bestored in the internal memory of the controller 20, entered into thecontroller 20 by a user, or provided to the controller by an externalsensor such as a temperature sensor. The operating temperature range forthe lens 30 will preferably be established based on the accuracy desiredfor the Z-axis measurements. Heating the lens 30 to the targettemperature range and maintaining the lens temperature within that rangewill ensure the focus will remain constant whether the light source 22has recently been turned on, remains on for a long period, or is turnedoff, so long as the entire range is set to be greater than the maximumoperating temperature of the light source 22.

In FIG. 5, at block 100, the temperature sensor 16 of embodimentstransmits a temperature measured proximately to lens 30 or signalrepresentative thereof to the controller 20. At block 110, thecontroller 20 preferably compares the temperature or signal receivedfrom the sensor 16 to a pre-determined minimum temperature that isstored in the internal memory 80 of the controller 20. If thetemperature received from the sensor 16 is less than the range minimum,then at block 120 the controller 20 of embodiments increases current tothe heating element 18, after which the system returns to block 100. Ifthe temperature received from the sensor 16 is greater than the rangeminimum, then at block 130 the controller 20 of embodiments compares thetemperature received from sensor 16 to a pre-determined maximumtemperature that is also stored in the internal memory 80 of thecontroller 20. If the temperature received from the sensor 16 is greaterthan the range maximum, then at block 140 the controller 20 ofembodiments decreases current to the heating element 18, after which thesystem returns to block 100 to perform another temperature measurement.If the temperature received from the sensor 16 is less than the rangemaximum, then at block 150 the controller 20 of embodiments maintainsthe current to the heating element 18 at its present level, and thecontroller 20 returns to block 100 to receive another temperaturemeasurement.

In accordance with another aspect of this invention, the illuminator 22comprises a plurality of individual light sources are arranged insegments as shown by the dotted lines 23 in FIG. 6. In some situations,for example to reduce or eliminate shadows, it is desirable toilluminate some but not all of the segments of light emitting diodeswhile leaving the other segments dark. This partial illuminationenhances the ability to inspect the object under test but results inuneven heating of the lens and its supporting structure. In accordancewith another aspect of this invention, the heater is provided that isalso divided into segments 27 preferably corresponding to the segmentsof the illuminator.

As shown in FIG. 7, a control system is provided for energizing theheater segments 66, 68, 72 corresponding to the temperature sensors 64,62, and 60 respectively of the illuminator. Only four segments are shownfor simplicity, it being understood that a greater number of segmentsmay be provided. In this way, the lens is heated relatively uniformlyeither by the heater or by the illuminator and various combinations ofilluminated and dark segments may be employed without unevenly heatingthe lens and supporting structure.

The segmented heater of this embodiment of the invention may becontrolled by a plurality of sensors 60, 62, 64 responsive to thetemperature of a plurality of segments of the lens and/or the supportingstructure to control the operation of the heaters to maintain a uniformtemperature. Alternatively, in accordance with a somewhat simplerembodiment of the invention, the heating elements are designed toprovide a heat output corresponding essentially to the heat output ofthe illuminator segments to which they correspond and a control systemis provided that energizes heating element when it's correspondingilluminator segment is dark and the deenergizes the heating element whenit's corresponding illuminator segment is on.

Turning for example now to FIG. 6, four light emitting diode segmentsare provided each occupying approximately 90° of the total area of theilluminator 56, and a four sensors 60, 62, 64, and 66 (not shown) areutilized, each of the sensors can measure the temperature of a segmentof the lens corresponding to a segment of the illuminator. The heatingelement is likewise divided into four segments, 66, 68, 72, and 73, notvisible in the drawing, each segment corresponding to and generallyaligned with one segment of the illuminator.

The controller 70 is connected to each of the sensors and each of theheaters and applies power to the heater segments corresponding to thedark sections of the illuminator and the sensors operate in a feedbackrelationship to maintain the temperature of each segment of the lens anda predetermined range.

While a variety of heating elements may be employed in connection withthis invention, an arrangement in which the heating elements areresistors or other elements that generate heat when power is applied tothem, arranged in the illuminator body is preferred. If resistors areemployed, the value is selected so that the heat produced by theresistor is approximately the same as the heat produced by thecorresponding segment of the illuminator when the LEDs are activated.

In accordance with another aspect of this invention, while a closed loopfor static control system may be employed, if the heating element isselected to accurately match the heat output of the LED taking thethermal resistance of the path between the LED and the lens as well asthe thermal resistance of the path between the heating element and thelens into account, a constant temperature may be employed by simplyensuring that one of but not both of the LED and heating elements isalways illuminated, that is, during the time that the LEDs, or at leastone of the LEDs is on, the heating element corresponding to each segmentof LEDs that is not on is activated. This will ensure that a relativelyconstant amount of heat is coupled to the lens thereby making thetemperature of the lens substantially uniform.

Referring again to FIG. 8, the controller 20 in this embodiments isoperatively connected to the heating elements 66, 68, 72 and 73 andtemperature sensors 60, 62, 64, and 66 by wire, although othertechniques for connecting the devices may be used, such as wirelesscommunications techniques. The temperature sensors 60, 62, 64 and 66preferably transmit a temperature measured proximately to lens 30 orsignal representative thereof to the controller 20 which, in turn,increases or decreases current to the heating elements 66, 68, 72 and 73as required to maintain the temperature of the lens 30 within a targettemperature range in accordance with methods disclosed herein.

We have found that in addition to stabilizing the refractive index ofthe lens by maintaining a constant temperature, the heater of thisinvention also minimizes any focus effects due to expansion orcontraction of the housing for the lens because of uneven heating.

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are or may be presently unforeseen may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed,and as they may be amended, are intended to embrace all suchalternatives, modifications, variations, improvements, and substantialequivalents. Further, the recited order of processing elements orsequences, or the use of numbers, letters, or other designationstherefore, is not intended to limit the claimed processes to any orderexcept as may be specified in the claims.

1. An optical system having focal length stabilization, the systemcomprising: a heat sensitive lens; a plurality of light sourcesthermally coupled to the lens and each heating different portions of thelens when each light source is on; a plurality of heating elementsthermally coupled to different portions of the lens; at least onetemperature sensor thermally coupled to the lens for sensing thetemperature of the lens either directly or indirectly; and a controllercommunicating with the at least one temperature sensor and the at leastone of the plurality of heating elements, the controller adjusting theheating element to maintain the lens temperature within a preselectedrange.
 2. The optical system of claim 1 in which the lens temperature isgreater than a reference temperature.
 3. The system of claim 2 in whichthe lens temperature is greater than a maximum temperature to which thelight source alone heats the lens.
 4. The system of claim 1 in which thecontroller maintains the lens temperature in each lens segment at asubstantially constant value that is greater than a maximum temperatureto which the light source would heat the lens segment whereby changes ina front focal length of the lens caused by temperature variation areminimized.
 5. The system of claim 4 further comprising an indicator thatis actuated when the lens temperature exceeds the maximum temperature towhich the light source would otherwise heat the lens.
 6. The system ofclaim 1 in which the lens is disposed within a housing and a lightsource surrounds a lower end of the housing.
 7. The system of claim 6 inwhich the light source comprises an annular lamp housing containing aplurality of radially spaced generally circular arrays of lamps.
 8. Thesystem of claim 7 wherein the lamps are light-emitting diodes.
 9. Thesystem of claim 1 in which the temperature sensor is a thermocouple. 10.The system of claim 1 in which the preselected range is programmed intothe controller.
 11. A heat sensitive lens focal length stabilizationmethod in which a lens is supported in thermal proximity to a pluralityof light sources each capable of heating a portion of the lenscomprising the steps of: monitoring the temperature of the lens in atleast one portion using a least one temperature sensor; determining apreselected lens temperature range; and maintaining the lens temperaturein the portion within the preselected range by controlling a heatingelement thermally coupled to the portion of the lens.
 12. The method ofclaim 11 wherein determining a preselected range comprises determining apreselected range that is greater than a reference temperature.
 13. Themethod of claim 11 in which determining a preselected range comprisesdetermining a preselected range that is greater than a maximumtemperature to which the light source would otherwise heat the lens. 14.The method of claim 11 in which maintaining the lens temperaturecomprises adjusting power to the heating element to maintain the lenstemperature at a temperature greater than a maximum temperature to whichthe light source could otherwise heat the lens, thereby minimizingchanges in a front focal length of the lens caused by temperaturevariation.
 15. The method of claim 11 comprising monitoring thetemperature of the lens in a morality of portions using a plurality oftemperature sensors.
 16. The method of claim 11 in which the step ofmaintaining the lens temperature comprises maintaining the temperaturewithin the preselected range in a plurality of portions.
 17. An opticalsystem having focal length stabilization, the system comprising: a heatsensitive lens; a plurality of light sources thermally coupled to thelens and each heating different portions of the lens when each lightsource is on; a plurality of heating elements each one corresponding toone of the plurality of light sources thermally coupled to saiddifferent portions of the lens; and a controller communicating with theplurality of light sources and the plurality of heating elements, thecontroller adjusting activating each one of the heating elements whenthe corresponding light source is off, to maintain the lens temperaturewithin a preselected range.